1. Field of the Invention
The present invention relates to high frequency amplifier circuits, and in particular, to high frequency amplifier circuits driven by a current signal source and providing high current gain.
2. Description of the Related Art
Referring to FIG. 1, cascode amplifiers (i.e., transistor Q2, driving by transistor Q1, in turn, drives transistor Q3 in a cascode configuration) are often used for video amplifier designs due to their ability to provide a high frequency signal response while also providing a moderate amount of voltage gain. For example, in a typical open loop cathode ray tube (CRT) video driver application, a cascode pair (transistors Q2 and Q3) can be driven by an emitter follower circuit (transistor Q1 and resistor R1) to create an amplifier with a high input impedance and a voltage gain in the range of 10-15.
However, some applications require significantly higher signal gains. In such applications, it is also sometimes necessary to operate with a differential input signal so as to minimize DC offsets, drift and noise. In still other applications, it may further be desirable for such a high bandwidth amplifier to receive its input signal in the form of a current signal.